1. Field of the Invention
The present invention concerns a channel estimation device.
2. Description of the Prior Art
In a transmission system such as a radio transmission system a transmitter sends a sequence of symbols to a receiver on a transmission channel. The sequence is subject to deterioration in the transmission channel with the result that the sequence of symbols received by the receiver is no longer identical to the sequence transmitted. The main deterioration is intersymbol interference caused by the fact that a symbol can take more than one path in the transmission channel. If at least two paths have a time difference exceeding the distance between two symbols transmitted in succession a symbol on one of these paths will interfere with a following symbol on another, shorter path.
An equalizer is used to correct intersymbol interference in the receiver. To operate correctly it must know the impulse response of the transmission channel. To this end special symbols are transmitted in a learning sequence. These are known symbols, unlike the data symbols transmitted which can be assumed to be unknown to the receiver. It is therefore standard practise for a packet of symbols sent to a specific receiver to comprise data symbols, a learning sequence and further data symbols, i.e. the learning sequence in the middle of the packet.
The learning sequence is chosen to suit the characteristics of the transmission channel and in particular its length.
Given that the symbols are transmitted regularly at a period called the symbol duration the length of the channel is defined as the number of symbol durations which is equal to the difference between the longest and shortest paths.
A channel estimator is used in the receiver to establish the impulse response of the transmission channel. It generates a replica of the learning sequence and correlates it with the received sequence of symbols. The result of this correlation is a set of coefficients h.sub.i with i varying from 0 through L where L is the length of the channel. This set of coefficients is supplied to the equalizer. The most direct path in the channel is represented by h.sub.0 and the other coefficients represent longer paths which cause interference with the first.
In the most usual case the coefficients are complex because the received symbols comprise "in-phase" and "quadrature" components which are orthogonal. Hereinafter the set of coefficients is called the impulse response.
To allow for the most diverse transmission conditions the length L of the channel would theoretically be infinite but in practise it is a constant chosen to suit the transmission system and supplied to the channel estimator. If this length is assigned a low value the longest paths of the channel are deliberately eliminated and the performance of the equalizer is degraded if these paths are actually used because the equalizer does not have all of the data representing the transmission channel. It would therefore seem desirable to use a high value for the channel length. However, this considerably increases the complexity of the equalizer, whether operating on the principle of symbol by symbol detection (for example the "Decision Feedback Equalizer" or DFE) or on the principle of maximum likelihood, by estimating a sequence of symbols (for example the "Viterbi" equalizer). The complexity of the equalizer, equivalent in the present context to the number of operations to be effected, is directly related to the number of correlation coefficients.
The patent application WO 92/11 708 proposes to eliminate all the coefficients whose modulus is less than a given fraction of the largest modulus of these coefficients.
This solution is directed to the specific instance of a channel having a profile in which the only meaningful paths correspond to coefficients which have substantially the same modulus, all the other coefficients representing paths which do not contribute any additional information. This solution is not generally applicable.
An object of the present invention is therefore a channel estimation device which, using a large channel length, enables the equalizer that it feeds to be simplified, regardless of the channel profile.